Variable Focal Backlighting

ABSTRACT

A backlight unit includes a waveguide assembly having a first wedge and a birefringent wedge disposed adjacent to, and arranged nose-to-tail with, the first wedge, to define an interface of the waveguide assembly. The birefringent wedge has different indices of refraction for light propagating through the waveguide assembly in first and second polarization states. The first wedge is configured to propagate the light in the second polarization state at a different speed than the birefringent wedge. A liquid crystal layer is configured to selectively switch the light between the first and second polarization states.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of co-pending U.S. patentapplication Ser. No. 14/476,896, entitled “Variable Focal Backlighting”and filed on Sep. 4, 2014, the entire disclosure of which is herebyincorporated by reference.

DESCRIPTION OF THE DRAWING FIGURES

For a more complete understanding of the disclosure, reference is madeto the following detailed description and accompanying drawing figures,in which like reference numerals may be used to identify like elementsin the figures.

FIG. 1 is a schematic view of a backlight unit configured for variableconvergence of backlight illumination in accordance with one example.

FIG. 2 is a schematic view of the backlight unit of FIG. 1 afteradjustment of an extent to which the backlight illumination isconverging in accordance with one example.

FIG. 3 is a schematic view of another backlight unit configured forvariable convergence of backlight illumination in accordance with oneexample.

FIG. 4 is a block diagram of an electronic device with a backlight unitconfigured for variable convergence of backlight illumination inaccordance with one example.

FIG. 5 is a schematic view of a light source of a backlight unit inaccordance with one example.

FIG. 6 is a schematic, perspective view of a volume hologram arrangementof a backlight unit in accordance with one example.

FIG. 7 is a flow diagram of a computer-implemented method of operatingan electronic device having a backlight unit with variable convergenceof backlight illumination in accordance with one example.

FIG. 8 is a block diagram of a computing environment in accordance withone example for implementation of the disclosed methods, devices, andsystems, or one or more components or aspects thereof.

While the disclosed devices, systems and methods are susceptible ofembodiments in various forms, specific embodiments are illustrated inthe drawing (and are hereafter described), with the understanding thatthe disclosure is intended to be illustrative, and is not intended tolimit the invention to the specific embodiments described andillustrated herein.

DETAILED DESCRIPTION

Display modules may include a display, for example, a liquid crystaldisplay, a backlight unit, and other components. Displays and backlightunits may be configured for variable convergence of backlightillumination. With variable convergence, the displays are capable ofcontrolling the angles at which the rays of the backlight illuminationconverge. Controlling the angles at which the backlight illuminationconverges allows the illumination to be focused at an appropriatedistance from the display. The appropriate distance may correspond withthe pupil location(s) of the viewer(s) of the display. The illuminationmay thus be concentrated at the pupil locations rather than at other,non-relevant distances. The displays may thus avoid emitting light notreceived by the pupils of a viewer. To these ends, the pupil locationsmay be tracked as the pupils move, for instance, forward and backward,as well as from left to right. The convergence of the backlightillumination is varied as the pupil locations change. As a result, lesslight is wasted. Lowering the amount of wasted light may thus reducepower consumption.

The convergence of the backlight illumination is varied via a waveguideassembly having a birefringent wedge disposed in a nose-to-tailarrangement with another wedge. The other wedge may be an isotropicwedge or other wedge that propagates light at a different speed than thebirefringent wedge for at least one polarization state. In some cases,the other wedge may be a birefringent wedge of the same or differentmaterial as the other birefringent wedge. If the two birefringent wedgesare composed of the same material, then the wedges are configured suchthat the axes of birefringence are orthogonal to one another.

A liquid crystal layer is disposed adjacent the waveguide assembly toselectively switch the polarization state of the light reflectivelypropagating down the waveguide assembly. Switching the polarizationstate is used to control the extent to which a given ray of the light isrefracted at the interface between the wedges. Selectively controllingthe refraction is used to control the path taken by a ray within thewaveguide assembly, which, in turn, establishes a range of angles atwhich rays of the light are emitted from the backlight unit. The rangeof angles may be adjusted to vary the distance at which the illuminationconverges. The distance at which the illumination is focused may thus bevaried as the pupil location changes.

The backlight units may also adjust the directionality of the backlightillumination. For instance, the focus of the backlight illumination maybe adjusted laterally as the pupil location(s) move left to right. Thedirectionality of the illumination may be adjusted by controlling theangle at which the light is injected into the waveguide assembly. Theillumination may thus be steered in addition to varying the angles atwhich the illumination converges. Power savings may thus be achieved byadjusting the directionality of the illumination focus.

The power consumption of a backlit display may be greatly reducedthrough configuring the illumination to include only those rays thattravel to the eyes of the viewer. A flat panel display may use as muchas one-third to one-half of the total power consumed by an electronicdevice. The backlight unit of the display may be responsible for aboutas much as 80% of that amount. The power savings may thus be useful inconnection with battery-powered devices, such as mobile phones,smartphones, tablets, wearable devices—including wrist-worn devices andhead-mounted devices—and laptop computers. Focusing the illumination mayallow the display to emit less total light for given ambient conditions.Focusing the illumination may also increase the ability of the backlightunit to make the display viewable in bright ambient conditions. A highlyemissive backlight may be provided without consumption of excessivepower. The fraction of the overall power budget for handheld, wearable,and other small devices dedicated to the display may thus be reduced.The size and weight of the batteries used in such devices may be reducedaccordingly. Alternatively or additionally, the devices may be operatedwith less frequent recharges. The reduction in power consumption may, inturn, reduce unwanted device heating.

Although useful in connection battery-powered devices, the backlightunits and displays may be used with a wide variety of electronicdevices. Examples of other devices include desktop computers andtelevisions. The size of the display may thus vary. Any electronicdevice having an image-forming liquid crystal display (LCD) or otherbacklit display may benefit from the variable convergence and othersteering.

The variable convergence of the backlight illumination may be useful inways in addition to those related to power savings. For instance, thevariable convergence may be useful in connection with providingthree-dimensional (3D) displays. Through variable convergence, one imagemay be focused on one pupil, while another image is focused on the otherpupil.

FIGS. 1 and 2 depict a backlight assembly 100 configured in accordancewith one example. The backlight assembly 100 may be used to illuminate aliquid crystal display (LCD). For example, the backlight assembly 100may be disposed behind an image-forming LCD panel of a display. Thecomponents, configuration, and other characteristics of the display mayvary. For example, the display may be configured as a 3D display.

The backlight assembly 100 includes a light source 102 and a waveguideassembly 104 into which light from the light source 102 is injected. Inthis example, the light source 102 is disposed at an end 106 of thewaveguide assembly 104. The end 106 of the waveguide assembly 104 isdisposed between a front face 108 and a rear face 110 of the backlightassembly 100. The light source 102 may be disposed at the end oppositethe end 106. Light extracted from the waveguide assembly 104 is emittedthrough the front face 108 of the backlight assembly 100. Othercomponents of the display, such as the image-forming LCD panel, may bedisposed along or otherwise in front of the front face 108.

The light source 102 may include one or more lasers, light emittingdiode (LED) devices, or other types of light-generating devices. Thelight source 102 may generate linearly polarized light and/or include alinear polarizer. The light may be generated at one wavelength, multiplewavelengths, or across one or more ranges of wavelengths. Alternatively,multiple light-generating devices may be used to provide light ofmultiple colors (e.g., red, green, and blue). In multiple wavelengthexamples (e.g., with the light source 102 generating white light), thelight source 102 may include one or more gratings configured to reverseor otherwise compensate for chromatic aberration introduced inconnection with diffraction out of the waveguide assembly 104.

The light source 102 may be controlled to adjust the angle at which thelight is injected into the waveguide assembly 104. In some cases, thelight source 102 includes one or more adjustable mirrors and/or otheroptical components to adjust the injection angle. The configuration,construction, and other characteristics of the light source 102 mayvary. For example, the manner in which the injection angle is adjustedmay vary.

The waveguide assembly 104 includes a pair of wedges 112, 114 disposedadjacent to one another in a nose-to-tail arrangement. In the example ofFIGS. 1 and 2, the thicker end of the wedge 112 is disposed at the end106 of the waveguide assembly 104 with the thinner end of the wedge 114.The wedges 112, 114 may or may not be identically shaped. For example,the wedges 112, 114 may be shaped differently at the end of thewaveguide assembly 104 opposite the end 106.

The nose-to-tail arrangement defines an interface 116 between the wedges112, 114. The interface 116 may be descending or ascending as the lightpropagates down the waveguide assembly 104. For example, the slope ofthe interface 116 may vary based on the end 106 at which the lightsource 102 is disposed. Alternatively or additionally, the relativepositions of the wedges 122, 114 and/or the front and rear faces 108,110 may be swapped.

The wedges 112, 114 are configured such that refraction may occur at theinterface 116. The extent to which refraction occurs as the lightpropagates down the waveguide assembly 104 is dependent upon thepolarization state of the light. At least one of the two wedges 112, 114is composed of a birefringent material to establish this dependency. Thebirefringent material has two different refractive indices depending onthe polarization of the light and the axis of birefringence of therespective wedge 112, 114. For instance, the refractive index may differfor light linearly polarized in, e.g., a vertical direction (verticalpolarization) from light linearly polarized in, e.g., a horizontaldirection (horizontal polarization). Examples of suitable birefringentmaterials include polymerized liquid crystal materials. In one example,the refractive indices of the birefringent material are about 1.5 andabout 1.8, but other refractive indices may be used. As a result of thebirefringence, light propagating through the respective wedge 112, 114travels at one of two different speeds depending upon the polarizationstate of the light.

In the example of FIGS. 1 and 2, the wedge 114 is composed of abirefringent material. The wedge 114 thus has different indices ofrefraction and, thus, different propagation speeds, for lightpropagating in two orthogonal polarization states.

The wedge 112 is configured to propagate light in at least one of thetwo orthogonal polarization states at a different speed than thebirefringent wedge 114. In some cases, the wedges 112, 114 are composedof different materials to establish the different speeds. For example,the wedge 112 may be composed of an isotropic material. In such cases,the refractive index of the wedge 112 is thus not dependent upon thepolarization state of the light propagating through the waveguideassembly 104. Examples of suitable isotropic materials include polymethyl methacrylate and other transparent materials. In one example, therefractive index of the isotropic material is about 1.5, but otherrefractive indices may be used. The refractive index of the isotropicmaterial may be equal to or about equal to one of the refractive indicesof the birefringent material of the wedge 114.

The wedge 112 may alternatively be composed of a birefringent material.In some cases, different birefringent materials are used in the wedges112, 114. In these cases, one or both of the refractive indices of thewedge 112 may differ from the refractive indices of the material usedfor the wedge 114. For example, the light of one orthogonal polarizationstate may travel faster if the light were in the orthogonal polarizationstate in one wedge and slower than if the light were in the orthogonalpolarization state in the other wedge. In other cases, the wedges 112,114 are composed of the same birefringent material. In such cases, thewedges 112, 114 may be oriented or configured such that the respectiveaxes of birefringence of the wedges 112, 114 are orthogonal to oneanother. Notwithstanding the option to use a birefringent wedge for thewedge 112, the wedge 112 may be referred to hereinafter as an isotropicwedge for ease in description.

At least one of the refractive indices of the birefringent material isoffset from the refractive index of the isotropic wedge 112. In theexamples of FIGS. 1 and 2, one of the refractive indices of thebirefringent wedge 114 matches the refractive index of the isotropicmaterial (i.e., the isotropic refractive index). As a result, when thelight has the polarization state corresponding with the matchingrefractive index (e.g., vertical polarization), the light does notundergo refraction at the interface 116. FIG. 1 depicts an exemplary ray118 of light having a polarization state for which the refractiveindices of the wedges 112, 114 are matching. No refraction occurs at theinterface 116, and the ray 118 of light propagates down the waveguideassembly 104 with unvarying reflection angles.

The polarization states may correspond with s-polarization andp-polarization. Other polarization states may be used. For example,various polarization orientations may be used. For instance, the statesmay be indicative of other pairs of orthogonal linear polarizations(e.g., oriented along diagonals).

FIG. 2 depicts an exemplary ray 120 of light having the otherpolarization state (e.g., horizontal polarization). In this state, theray 120 experiences a refractive index of the birefringent materialoffset (e.g., substantially offset) from the isotropic refractive index.For example, the effective refractive index of the birefringent wedge114 may be about 1.8, while the refractive index of the isotropic wedge112 may be about 1.5. The ray 120 thus undergoes refraction at eachcrossing of the interface 116 so long as the polarization state remainsunchanged. In that state, the ray 120 travels through the waveguideassembly 104 as if encountering a series of prisms. With each encounter,the angle of propagation gradually diverges from the normal to theboundary of the waveguide assembly 104.

In other cases, both of the refractive indices of the birefringentmaterial are offset from the isotropic refractive index. One of therefractive indices of the birefringent material is more offset from theisotropic refractive index than the other refractive index of thebirefringent material is offset from the isotropic refractive index. Inone example, one of the refractive indices of the birefringent materialdoes not exactly match the isotropic refractive index. For instance, therefractive index for horizontally polarized light may be near, but notequal to, the isotropic refractive index. In such cases, thehorizontally polarized light may thus be slightly refracted, whilevertically polarized light is more refracted (e.g., substantially morerefracted). In still other examples, neither of the refractive indicesof the birefringent material is near the isotropic refractive index. Thelight may thus undergo substantial or significant refraction at theinterface 116 regardless of polarization state. But the amount ofrefraction is still dependent upon the polarization state in such cases.

With reference to FIGS. 1 and 2, the backlight assembly 100 includes anextraction grating 122 to extract light from the waveguide assembly 104.The extraction grating 122 is configured to frustrate the total internalreflection (TIR) that would otherwise cause all of the light topropagate down the waveguide assembly 104 until reaching the endopposite from the end 106. The pitch of the extraction grating 122 maybe sufficiently short such that, once the light leaves the first order,the light is forced to leave the waveguide assembly 104 at a specificangle. The extraction grating 122 extends laterally along the waveguideassembly 104. For example, the extraction grating 122 may extend theentire length (and/or width) of the waveguide assembly 104. Theextraction grating 122 may be configured to vary the amount ofextraction as a function of lateral position along the length of thewaveguide assembly 104. The extraction amount may increase with thedistance from the light source 102 so that the light may be uniformlyextracted from the waveguide assembly 104. The amount of extraction maybe increased to ensure that all or substantially all of the light isdiffracted out of the waveguide assembly 104 as the light reaches theend of the waveguide assembly 104.

In the example of FIGS. 1 and 2, the extraction grating 122 is disposedalong the birefringent wedge 114. The extraction grating 122 isalternatively disposed along the isotropic wedge 112. In this example,the extraction grating 122 is configured as a layer or film adjacent to(e.g., contiguous with) the waveguide assembly 104. The extractiongrating 122 may be embossed, bonded or otherwise secured to thewaveguide assembly 104. The structure, configuration, or othercharacteristics of the extraction grating 122 may vary. For example, asurface ripple with an amplitude of about 0.1 microns and a pitch of 2.4ripples per micron may be used as the extraction grating 122, but theamplitude and pitch may vary. In other cases, the extraction grating 122is formed on or in the waveguide assembly 104. The extraction grating122 may be recorded in the film, layer, or waveguide assembly 104 as aBragg grating or other volume hologram, but other types of gratings maybe used.

Light is extracted by the extraction grating 122 at an emission angleestablished in accordance with an angle of incidence of the light on theextraction grating 122. For example, in FIG. 1, a set of parallel rays124-127 are emitted because the angle of incidence remains the same.Each ray 124-127 may be diffracted at the interface with the extractiongrating 122 as shown. The interface may, for example, be an airinterface. But the amount of diffraction is the same for each ray124-127. The rays 124-127 accordingly remain parallel.

In contrast, in FIG. 2, a set of converging rays 128-131 are emitted.The rays 128-131 are converging because of the refraction occurring whenthe ray 120 crosses the interface 116 within the waveguide assembly 104.The refraction changes the angle of incidence of the ray 120 on theextraction grating 122. The refraction also changes the angle at whichthe rays 128-131 are emitted (i.e., the emission angle) because theemission angle is established in accordance with the angle of incidence.The emission angle may be defined relative to the direction normal tothe surface of the backlight assembly 100. As the number of refractionsat interface crossings increases, the emission angle is decreasing. Theextent to which the emission angle decreases is determinative of therate at which the rays 128-131 are converging.

The angle of incidence and, thus, the emission angle are determined byan extent to which the ray 120 encounters the interface 116 whilepropagating through the waveguide assembly 104 in the polarization statefor which the refractive index of the birefringent material is offset(or most offset) from the isotropic refractive index. With eachencounter of the interface 116 in that polarization state, the emissionangle is changed. The amount of refraction and, thus, the extent towhich the emission angles change, may be less dramatic than as shown inFIG. 1. The drawings may exaggerate the amount of refraction for ease inillustration. In practice, the angle of incidence of the ray 120 mayalso be limited by the critical angle(s) for the waveguide assembly 104.

The amount of diffraction at the interface with the extraction grating122 may vary based on the material of the waveguide assembly 104 (and/orextraction grating 122) and the material on the other side of theinterface, as well as the pitch of the extraction grating 122. In oneexample, the extraction grating 122 is configured such that a rayencountering the extraction grating 122 at an angle of incidence halfwaybetween the critical angle of the waveguide assembly 104 (e.g., 42degrees in acrylic) and an angle above which any light produced from thebacklight unit 100 will not be of much use (e.g., about 72 degrees)produces diffracted components traveling perpendicular to the waveguideassembly 104 (i.e., in the direction of the surface normal). In anacrylic example, the halfway angle is about 57 degrees. With thishalfway point, during operation, angles greater than about 57 degreesproduce diffracted components that are off-normal in the same sense asthe ray propagating through the waveguide assembly 104. Angles less thanabout 57 degrees produce diffracted components that are off-normal inthe opposite sense as the ray propagating through the waveguide assembly104. The halfway point angle may vary in other examples.

With continued reference to FIGS. 1 and 2, the backlight assembly 100includes a liquid crystal layer 132 extending along the waveguideassembly 104. In this example, the liquid crystal layer 132 is disposedadjacent to (e.g., contiguous with) the isotropic wedge 112. The liquidcrystal layer 132 may alternatively be disposed adjacent to (e.g.,contiguous with) the birefringent wedge 114. The liquid crystal layer132 is configured to selectively switch the polarization of the ray 120of light as the ray 120 reflectively propagates through the waveguideassembly 104. To that end, the thickness of the liquid crystal layer 132may be selected such that, when activated, the liquid crystal layer 132acts as a quarter-wave plate. An exemplary thickness may be about 1.5microns, but may vary based on material and/or other factors. Each timethe ray 120 reflects at the rear face 110 within the area of anactivated pixel 134, the ray 120 passes through the liquid crystal layer132 twice (i.e., before and after reflection). The ray 120 accordinglyexperiences a half-wave plate. The activated pixel 134 may thuseffectively rotate the polarization 90 degrees. To that end, the liquidcrystal layer 132 may be oriented or otherwise configured such that theprincipal axes of the (effective) half-wave plate are offset 45 degreesfrom the plane of the transmission axis of the linear polarization ofthe ray 120. If the ray 120 passes through the (effective) half-waveplate, the polarization state of the light is rotated 90 degrees,thereby toggling the polarization of the ray 120 from one state to theother. The liquid crystal layer 132 may thus be used to switch betweenvertical and horizontal polarization states (or between other orthogonalpolarization states). The liquid crystal layer 132 may thus becontrolled to, in turn, control the rate at which the angle of incidenceof the ray 120 is adjusted during propagation down the waveguideassembly 104.

The liquid crystal layer 132 includes an array of pixels 134 disposedalong the length of the waveguide assembly 104. The array of pixels 134are used to selectively control the extent to which the ray 120 isrefracted at the interface 116 along the length of the waveguideassembly 104. The pattern of pixel activation may thus support avariable continuum of backlight illumination convergence.

In the example of FIGS. 1 and 2, six pixels are depicted for ease inillustration. The array may include any number of pixels 134. Each pixel134 is arranged in the liquid crystal layer 132 to have, as describedabove, a thickness such that each pixel 134 is configured to act as aquarter-wave plate when activated. Each pixel 134 may thus act as ahalf-wave plate with the light passing through twice due to reflection.Each pixel 134 is separately controlled to provide the selectivepolarization switching. Each pixel 134 is activated or not activated todetermine whether the polarization state of the ray 120 is toggled orswitched in the event that the ray 120 encounters the pixel 134. The ray120 thus remains in its current polarization state until encounteringanother activated pixel 134. The polarization of the ray 120 may thus becontrolled on a pixel-by-pixel basis.

An example of the selective polarization switching of the pixels isshown in FIG. 2. In this example, only pixels 134A and 134B areactivated. The light source 120 is configured to generate linearlypolarized in a direction (e.g., horizontal polarization) such that theray 120 is refracted at the interface 116. The first two reflections ofthe ray 120 encounter one of the first two pixels 134. Because each ofthose pixels 134 is not active, the polarization state of the ray 120remains the same (e.g., horizontal polarization) after each of thosereflections. The ray 120 then encounters the pixel 134A. Because thepixel 134A is active, the ray 120 assumes the other polarization state(e.g., vertical polarization) after the reflection. As a result, the ray120 does not experience a difference in refractive index upon reachingthe interface 116, and the ray 120 passes through the interface 116without refraction. The ray 120 encounters the interface 116 afterreflection at the front face 108 again without refraction. The nextreflection at the liquid crystal layer 132 toggles the polarizationstate back to the original state due to the activation of the pixel134B. The ray 120 is accordingly then refracted at the interface 116.Further refraction, polarization switching, and reflection may thencontinue as the ray 120 reflectively propagates down the waveguideassembly 104.

The nature of the offsets between the refractive indices may vary. Forexample, the single refractive index of the isotropic material of theisotropic wedge 112 may be higher or lower than both of the refractiveindices of the birefringent wedge 114. Alternatively, the isotropicrefractive index may be higher than one of the birefringent indices, butlower than the other birefringent index.

The positions of the wedges 112, 114 may be switched. The wedges 112,114 are configured such that the extent to which refraction occurs asthe light reflectively propagates down the waveguide assembly 104 isdependent upon the polarization state of the light.

The positions of the extraction grating 122 and the liquid crystal layer132 may differ from the example of FIGS. 1 and 2. For instance, theextraction grating 122 may be disposed along (e.g., adjacent to, orcontiguous with) the liquid crystal layer 132 rather than thebirefringent wedge 114. The liquid crystal layer 132 may be disposedalong (e.g., adjacent to, or contiguous with) the birefringent wedge 114rather than the isotropic wedge 112. These alternative configurationsmay be used with or without changing the positions of the extractiongrating 122 and the liquid crystal layer 132 relative to the front andrear faces 108, 110.

In some cases, the backlight assembly 100 is configured as a primarybacklight unit for the display. The backlight assembly 100 may thus havea size that corresponds with the entire viewable area of the display.For example, the backlight assembly 100 may be panel-shaped. The end 106may thus extend along a side or edge of the panel. The front face 108 ofthe backlight assembly 100 may thus extend over the entire lateralextent of the display. For example, the front face 108 of the backlightassembly 100 may have roughly the same lateral dimensions as theimage-forming LCD panel. In those cases, the backlight assembly 100 mayinclude multiple light sources 102 disposed along the end 106.

The shapes of the respective components of the backlight assembly 100,such as the waveguide assembly 104, may vary in accordance with theshape of the backlight assembly 100. For example, if the backlightassembly 100 extends over the entire viewable area of the display, thenthe waveguide assembly 104 (and the components thereof) and othercomponents of the backlight assembly 100 may be plate- or panel-shaped.Alternatively, the waveguide assembly 104 and other components of thebacklight assembly 100 may be elongate or otherwise shaped.

The variability in the convergence provided by the backlight assembly100 may be sufficient for some displays and/or contexts. For example,the backlight assembly 100 may vary the convergence angles toaccommodate a range of focal distances well suited for viewingwall-mounted displays. In such cases, the rays emitted by the backlightassembly 100 may converge at a focal distance ranging from about severalfeet to about several yards or more. Convergence at closer positions maybe obtained by placing a lens (e.g., Fresnel lens) in between thebacklight unit 100 and the other components of the display. For example,the lens may be contiguous with or otherwise disposed adjacent to thefront face 108. The lens may allow the endpoints of the range ofavailable focal distances may thus be changed. For example, one endpointwill no longer be effectively infinity, as shown in FIG. 1, due toparallel rays, but rather a shorter focal distance in accordance withthe focal length of the lens.

Other displays or contexts may warrant further variability in theconvergence. For instance, shorter focal distances may be appropriatefor viewing displays of handheld and wearable electronic devices, or forviewing laptop computer displays. For example, focal distances formobile phones may range from about several inches to about one foot. Insuch cases, the convergence provided by the backlight unit 100 may beamplified or increased by subsequent optical processing. The raysemitted from the backlight unit 100 may thus emerge not into air, butrather into a further optical component or stage of the backlight. Thefurther optical component is configured to amplify the variability inthe convergence angles to increase the range of attainable convergenceangles. The further optical component may be a holographic or othercomponent.

FIG. 3 depicts an example in which a backlight unit 300 uses volumeholograms to amplify the variability of the convergence. The backlightunit 300 includes an elongate waveguide assembly 302 and a panel or slabwaveguide 304 adjacent the waveguide assembly 302. The elongatewaveguide assembly 302 may be configured in accordance with thewaveguide assembly 104 described in connection with FIG. 1. Thewaveguide assembly 302 may thus include an isotropic wedge and abirefringent wedge arranged nose-to-tail with the isotropic wedge. Theisotropic refractive index of the isotropic wedge and the refractiveindices of the birefringent wedge may differ as described above.

In operation, the waveguide assembly 302 injects light into the panelwaveguide 304 over a range of emission angles. The panel waveguide 304includes an arrangement of volume holograms responsive to the emissionangles to provide illumination at predetermined angles for convergenceat a desired focal distance. The volume holograms are configured toamplify an extent to which the light extracted from the waveguideassembly 302 is converging. The light propagates through the panelwaveguide 304 under total internal reflection until encountering one ofthe volume holograms corresponding to the reflection angle at which thelight is propagating.

The panel waveguide 304 may be composed of acrylic, polycarbonate,glass, or other transparent materials. The panel waveguide 304 may beshaped and sized to extend over a viewable area 306 of the display. Thewaveguide assembly 302 extends across the panel waveguide 304 along anend or edge 308 of the panel waveguide 304. In this example, thewaveguide assembly 302 is adjacent a front face of the panel waveguide304. The positioning of the waveguide assembly 302 may vary. Forexample, the waveguide assembly 302 may be disposed along a rear face ofthe panel waveguide 304. An exploded view of the backlight unit 300 isshown in FIG. 3 to better depict the configuration of the panelwaveguide 304 and to show the injection of light into the panelwaveguide 304.

The elongate waveguide assembly 302 may cover an end region along theedge 308 outside of the viewable display area 306. The elongatewaveguide assembly 302 may be narrowly shaped to minimize the size ofthe end region. For example, the elongate waveguide assembly 302 mayhave a width of about 1 mm. The width may vary with the overall lateraldimensions of the backlight unit 300. The narrow shape of the waveguideassembly 302 may maximize the area over which the backlight unit 300provides illumination and, thus, the viewable display area 306. Theelongate waveguide assembly 302 may be band- or strip-shaped. Thethickness of the elongate waveguide assembly 302 may roughly correspondwith the thickness of other components of the display stacked in frontof the panel waveguide 304, such as an LCD panel and/or a transparentcover.

The volume hologram arrangement of the panel waveguide 304 includes anarray 310 of volume holograms distributed over the viewable display area306. The panel waveguide 304 also includes a set 312 of turningstructures distributed laterally across the slab waveguide in the endregion along the edge 308. The elongate waveguide assembly 302 isdisposed at the end region such that rays 314 of light extracted atspecific emission angles are emitted into the panel waveguide 304 forinteraction with the set 312 of turning structures. The set 312 ofturning structures redirect the light toward the array 310 of volumeholograms at a panel propagation angle based on the emission angle atwhich the light is injected into the panel waveguide 304. Each volumehologram of the array 310 is configured to emit backlight from the panelwaveguide at a convergence angle in accordance with the panelpropagation angle and the emission angle.

In the example of FIG. 3, the set 312 of turning structures includes aset 312 of volume holograms responsive to the emission angle. Eachvolume hologram of the set 312 is activated at only one injection angle(i.e., the emission angle from the elongate waveguide assembly 302).When activated, the volume hologram is configured to redirect the lightdown the panel waveguide 304 at a unique angle (e.g., a unique anglerelative to a normal to the front surface of the panel waveguide 304).In some examples, the set 312 of turning structures may include about 20or more volume holograms, but other set sizes may be used.

The volume holograms of the array 310 are configured such that only asingle volume hologram is activated by light at each unique angle. Theactivation of one of the volume holograms causes a respective ray 316 oflight to be emitted from the panel waveguide 304 at a predeterminedangle. The volume holograms of the array 310 may be recorded such thatthe rays 316 are converging more quickly than the rays 314 emitted fromthe elongate waveguide assembly 302, but still at convergence anglesdetermined in accordance with the emission angles of the rays 314.

The backlight unit 300 includes a number of other components directed toinjecting the rays 314 of light into the panel waveguide 304. Thecomponents may be configured similarly to corresponding componentsdescribed above in connection with FIGS. 1 and 2. For example, anextraction grating 318 may be embossed or otherwise secured or formed onthe waveguide assembly 302. The extraction grating 318 is configured toextract light propagating down the waveguide assembly 302 at an emissionangle in accordance with the internal reflection angles, as describedabove. A liquid crystal layer 320 is disposed along the waveguideassembly 302 to control the selective refraction of the light within thewaveguide assembly 302, as described above. In the example of FIG. 3,the liquid crystal layer is disposed along the face of the waveguideassembly 302 closest to the panel waveguide 304, while the extractiongrating 318 is disposed on the opposite face. The positions may beswitched or otherwise differ in other embodiments. Light may be providedto the waveguide assembly 302 by one or more light sources 322 disposedat an end of the waveguide assembly 302.

FIG. 4 shows an exemplary electronic device 400 with a display system402 (or subsystem) configured for variable convergence backlighting. Thedisplay system 402 may be integrated with other components of theelectronic device 400 to a varying extent. The display system 402 may beor include a graphics subsystem of the electronic device 400. Any numberof display systems may be included. In this example, the device 400includes a processor 404 and one or more memories 406 separate from thedisplay system 402. The processor 404 and the memories 406 may bedirected to executing one or more applications implemented by the device400. The display system 402 generates a user interface for an operatingenvironment (e.g., an application environment) supported by theprocessor 404 and the memories 406. The processor 404 may be ageneral-purpose processor, such as a central processing unit (CPU), orany other processor or processing unit. Any number of such processors orprocessing units may be included.

The display system 402 may be communicatively coupled to the processor404 and/or the memories 406 to support the display of video or otherimages via the user interface. For example, the processor 404 mayprovide frame data indicative of each image frame to the display system402. The frame data may be generated by the processor 404 and/or byanother component of the device 400. The frame data may be alternativelyor additionally obtained by the processor 404 from the memory 406 and/oranother component of the device 400.

In the example of FIG. 4, the display system 402 includes a processor408, one or more memories 410, firmware and/or drivers 412, a backlightunit (BLU) 414, and an LCD panel 416. The processor 408 may be agraphics processing unit (GPU) or other processor or processing unitdedicated to graphics- or display-related functionality. Some of thecomponents of the display system 402 may be integrated. For example, theprocessor 408, one or more of the memories 410, and/or the firmware 412may be integrated as a system-on-a-chip (SoC) or application-specificintegrated circuit (ASIC). The display system 402 may includeadditional, fewer, or alternative components. For example, the displaysystem 402 may not include a dedicated processor, and instead rely onthe CPU or other processor 404 that supports the remainder of theelectronic device 400. The display system 402 may not include the memory(or memories) 410, and instead use the memories 406 to supportdisplay-related processing. In some cases, instructions implemented by,and data generated or used by, the processor 408 of the display system402 may be stored in some combination of the memories 406 and thememories 410.

The backlight unit 414 may be configured for variable convergencebacklighting as described above. In addition to the waveguide-relatedcomponents, the backlight unit 414 includes one or more light sources418 and a liquid crystal layer 420 to selectively switch thepolarization state of the guided rays, as described above. The lightsources 418 and the liquid crystal layer 420 may be controlled by theprocessor 408 via, for example, the firmware or drivers 412. The lightsources 418 are controlled to adjust the directionality of thebacklighting, while the pixels of the liquid crystal layer 420 arecontrolled to adjust the focus distance, or convergence, of thebacklighting.

The liquid crystal layer 420 of the backlight unit 414 is distinguishedfrom the LCD panel 416. The LCD panel 416 is controlled and configuredto form images in accordance with the frame data provided by, forinstance, the processor 404. The LCD panel 416 is disposed to receivethe illumination from the backlight unit 414. In contrast, the liquidcrystal layer 420 is controlled to vary the extent to which theillumination provided by the backlight unit 414 converges. The liquidcrystal layer 420 may thus be referred to as a focusing (or focusadjustment) liquid crystal layer. The extent to which the liquid crystallayer 420 adjusts the focus distance or convergence of the backlightillumination is dependent upon the position of the viewer (or pupils ofthe viewer) rather than the image content represented by the frame data.

The display system 402 includes one or more cameras 422 for pupiltracking. Each camera 422 is oriented toward the viewer of the display.For example, when the electronic device 400 is a laptop computer, thecamera(s) 422 may include one or more rear-facing cameras positionedalong a display bezel. In some cases, the camera(s) 422 include one ormore depth cameras and/or one or more color cameras. The configurationof the camera(s) 422 may vary. The camera data captured by the displaysystem 402 may be directly or indirectly indicative of pupil position.For example, the camera data may be processed to resolve one or morevarious features of the viewer's eye, such as the center of the pupil,the outline of the pupil, the position of the iris, and/or one or morespecular glints from the cornea. The location data for one or more ofthese features may be used as input parameters of a model, e.g., apolynomial model, that provides pupil coordinates in a reference framerelative to the display system 402.

The LCD panel 416, the light source(s) 418, the focusing liquid crystallayer 420, the cameras 422 may be controlled by the processor 408. Theprocessor 408 may provide such control in accordance with a number ofinstruction sets stored in the memories 410. In this example,instruction sets 424-426 are provided for main LCD panel control, pupiltracking, and backlight unit (BLU) illumination control, respectively.The instruction set 424 may direct the processor 408 to drive the LCDpanel 416 in accordance with the frame data. The instruction set 425 maydirect the processor 408 to determine and track the position of viewerpupils. The pupil position data is then used by the processor 408 toadjust the directionality (steering) and convergence (focus) of thebacklight illumination set 424 in accordance with the instruction set426.

The pupil tracking instructions 425 may configure the processor 408 and,thus, the display system 402, to acquire a time-resolved sequence ofdepth maps. The term ‘depth map’ refers to an array of pixels registeredto corresponding regions of an imaged subject, with a depth valueindicating, for each pixel, the depth of the corresponding region.‘Depth’ is defined as the coordinate orthogonal to the plane of thebacklight unit 414, which increases with increasing distance from thecameras 418. In some cases, the display system 402 may be configured toacquire two-dimensional image data from which a depth map is obtainedvia downstream processing.

Other pupil tracking techniques may be used. For example, brightness orcolor data from two, stereoscopically oriented cameras 418 may beco-registered and used to construct a depth map. In other cases, a depthcamera may be configured to project onto the subject a structuredinfrared (IR) or near-IR (NIR) illumination pattern having numerousdiscrete features, such as lines or dots. An imaging array in the depthcamera 418 may be configured to image the structured illuminationreflected back from the viewer. A depth map of the viewer may beconstructed based on the spacing between adjacent features in thevarious regions of the imaged subject. In still other cases, the depthcamera may project a pulsed illumination toward the viewer. A pair ofimaging arrays may be configured to detect the pulsed illuminationreflected back from the viewer. Both arrays may include an electronicshutter synchronized to the pulsed illumination, but the integrationtimes for the arrays may differ, such that a pixel-resolvedtime-of-flight (TOF) of the pulsed illumination, from the illuminationsource to the viewer and then to the arrays, is discernible based on therelative amounts of light received in corresponding elements of thearrays. A TOF depth-sensing camera 418 that measures the phase shiftbetween transmitted and reflected light may also be used.

The LCD drive instructions 424, the pupil tracking instructions 425, andthe backlight illumination control instructions 426 may be arranged indiscrete software modules or instruction sets in the memories 410.Alternatively, two or more of the instructions or definitions 424-426may be integrated to any desired extent. The instructions or definitions424-426 may alternatively or additionally be integrated with otherinstructions, definitions, or specifications stored in the memories 410.Additional instructions, modules, or instruction sets may be included.For instance, one or more instruction sets may be included forprocessing touch inputs in cases in which the display system 402includes a touchscreen or other touch-sensitive surface.

The processing of the camera data and other aspects of the variablebacklighting techniques may be implemented by any combination of theprocessor 404, the processor 408, and/or one or more other processor(s),which may be collectively referred to as a processor. In other examples,the device 400 includes a single processor (i.e., either the processor404, the processor 408, or a different processor) for purposes ofprocessing the frame data, the camera data, and other data involved incontrolling the display system 402.

FIG. 5 depicts an exemplary light source 500 for use in connection withthe backlight units described herein. The light source 500 is configuredfor adjustment of the angle at which the light is injected into thewaveguide assembly to which the light source 500 is adjacent, such asthe waveguide assembly 104 of FIG. 1 or the waveguide assembly 302 ofFIG. 3. The injection angle determines the initial angle at which theguided rays encounter the extraction grating (e.g., the extractiongrating 108 of FIG. 1) and, thus, the emission angle. The initial anglemay be determinative of the general direction in which the light isemitted from the waveguide assembly without any convergence, as shown,for example, in FIG. 1. Subsequent adjustment of the reflection anglesmay then be used to adjust the extent to which the emitted raysconverge, as described above.

The light source 500 is configured to inject collimated (orsubstantially collimated) light into the waveguide assembly. The lightsource 500 may include or be configured as a laser. In the example ofFIG. 5, the light source 500 includes independently biased red, green,and blue lasers 502-504. Each laser 502-504 may be a diode laser. Inother examples, a single laser is used. In still other examples, LED orother light emitting devices are used. Thus, in some cases, the lightsource 500 may include a polarizer. The light may or may not becoherent. In this example, the light is passed through an expander 506.The expander 506 may be used to combine the beams from the individuallasers 502-504. In other cases, the light source 500 does not include anexpander.

The output from the expander 506 is directed to a mirror 508, which iscoupled to a piezoelectric mirror mount 510. In this example, a controlvoltage applied to a piezoelectric element of the mirror mount 510proportionally rotates the mirror 508 about an axis 512. The rotation ofthe mirror 508 may be used to control the injection angle. In otherexamples, the mirror mount 510 may include two piezoelectric elements tocontrol the deflection of the mirror in two, orthogonal directions. Instill other cases, the laser light may be deflected in orthogonaldirections by two different mirrors, each coupled to its ownpiezoelectric mirror mount. Non-piezoelectric mechanical transducers mayalternatively be used. Non-mechanical transducers may also be used, suchas an electronically tunable optic configured to deflect the laser lightby a controllable amount. Other micro-electromechanical devices may beused to implement the mirror 508 and mirror mount 510.

FIG. 6 depicts an exemplary volume hologram arrangement in a panelwaveguide 600. The panel waveguide 600 may be used in the backlight unit300 of FIG. 3. A similar arrangement may be used to provide the set 312of turning structures shown in FIG. 3. In this example, each volumehologram is established via a respective Bragg grating 602, 604. TheBragg gratings may be superimposed over the lateral extent of the panelwaveguide 600. Only two Bragg gratings are shown for ease inillustration. In practice, the panel waveguide 600 may have a muchhigher total number of Bragg gratings (or other volume holograms), suchas about 20 or more volume holograms. Each Bragg grating 602, 604provides a volume hologram for a respective range of angles. Becauseeach Bragg grating is excited by a very narrow range of angles (e.g.,about 1 degree), a large number of Bragg gratings (e.g., over 100) aresuperimposed within the panel waveguide to cover the entire range ofpossible angles (e.g., plus or minus about 60 degrees). The thickness ofthe Bragg gratings 602, 604 may fall in a range from about 10 microns toabout 100 microns, but other thicknesses may be used. The Bragg gratings602, 604 may vary with respect to orientation and pitch.

The respective excitation angles of each Bragg grating 602, 604 aredetermined by the orientation and wavelength of the light used to recordthe Bragg grating. Outside the appropriate wavelength band or range ofincidence angles, the Bragg grating is transparent. This characteristicallows numerous Bragg gratings to occupy the same space within the panelwaveguide 600 and nonetheless operate independently of each other. TheBragg gratings 602, 604 may thus extend the entire area of the panelwaveguide 600 and, are instead partially shown in FIG. 6 for ease inillustration.

Each Bragg grating 602, 604 or other volume hologram may be configured,when excited, to diffract a portion of the light propagating through thepanel waveguide 600 and to eject such light in a predetermined,different direction, which is selectable based on the manner in whichthe hologram is recorded. As shown in FIG. 3, the useful directions liein horizontal planes orthogonal to the plane of backlight unit 300 andspan a range of horizontal-plane angles. During the recording process,each angle is mapped to light of a different reflection angle within thepanel waveguide 600, which may range from the critical angle (42 degreesin polyacrylic) to a glancing angle of 5 degrees or less. Due to theaction of the Bragg gratings 602, 604, the injection angle at whichlight is injected into the panel waveguide 600 influences a reflectionangle at which the injected light reflects from the front and back facesduring propagation through the panel waveguide 600. The superposition ofthe Bragg gratings 602, 604 transforms the angle of injection into thewaveguide to an orthogonal emission angle out of the backlight unit.

FIG. 7 depicts an exemplary process for varying backlight illuminationconvergence. The method 700 is computer-implemented. For example, one ormore computers of the electronic device 100 shown in FIG. 1 and/oranother electronic device may be configured to implement the method or aportion thereof. The implementation of each act may be directed byrespective computer-readable instructions executed by the processor 408(FIG. 4) of the display system 402 (FIG. 4), the processor 404 (FIG. 4)of the device 400, and/or another processor or processing system.Additional, fewer, or alternative acts may be included in the method700. For example, the method 700 may include one or more acts directedto determining whether steering and/or convergence is possible given thecurrent pupil location.

The method 700 may be implemented concurrently with other display systemcontrol procedures, including, for instance, the processing of framedata to generate LCD drive signals. The method 700 may be implemented toupdate the backlight illumination parameters at a rate different thanthe other procedures. For example, the method 400 may be implementedmuch less frequently than the frame update rate. A lower update rate forthe backlight illumination parameters may be sufficient because pupillocation does not change that abruptly.

The method 700 may begin with one or more acts related to controllingone or more cameras to capture data indicative of the viewer of thedisplay. Camera data is received or otherwise obtained in act 702. Thecamera data may be received from multiple cameras, including, forinstance, an infrared or other depth camera. The nature of the cameradata may vary as described above.

The camera data is processed in act 704. In some cases, the act 704includes identification of pupils in act 706 and calculation of pupilcoordinates in act 708. The remainder of the method 700 may then beimplemented for each identified pupil. For example, the method 700 maythen be repeated, e.g., in a processing loop, for each identified pupil.

Once the location of the viewer's eyes is known, the lateral positiondata is used to determine light source injection angles in act 710. Thelight source injection angle is selected to steer the focal point of thebacklight illumination. For example, the backlight illumination may besteered left to right.

In act 712, a liquid crystal layer activation pattern is determinedbased on the depth of the pupil position. Pixels of the liquid crystallayer are selectively activated to control the extent to which theillumination converges.

Drive signals are then generated for the backlight unit in act 714 inaccordance with the parameters determined in the acts 710 and 712. Forexample, drive signals for the light source and the liquid crystal layerare generated.

The order of the acts of the method may vary from the example shown. Forexample, in some cases, the acts are implemented in parallel orconcurrently, such as while processing data in connection with multiplepupil locations. As another example, the liquid crystal layer patternmay be determined before or concurrently with the determination of thelight source injection angle.

With reference to FIG. 8, an exemplary computing environment 800 may beused to implement one or more aspects or elements of the above-describedmethods and/or systems and/or devices. The computing environment 800 maybe used by, incorporated into, or correspond with, the electronic device400 (FIG. 4) or one or more elements thereof. For example, the computingenvironment 800 may be used to implement one or more elements of theelectronic device 400. In some cases, the display system 402 (FIG. 4)may be incorporated into the computing environment 800.

The computing environment 800 may be a general-purpose computer systemor graphics- or display-based subsystem used to implement one or more ofthe acts described in connection with FIG. 7. The computing environment800 may correspond with one of a wide variety of computing devices,including, but not limited to, personal computers (PCs), servercomputers, tablet and other handheld computing devices, laptop or mobilecomputers, communications devices such as mobile phones, multiprocessorsystems, microprocessor-based systems, set top boxes, programmableconsumer electronics, network PCs, minicomputers, mainframe computers,audio or video media players, and wearable computers such as wrist-worndevices or head-mount devices.

The computing environment 800 has sufficient computational capabilityand system memory to enable basic computational operations. In thisexample, the computing environment 800 includes one or more processingunit(s) 810, which may be individually or collectively referred toherein as a processor. The computing environment 800 may also includeone or more graphics processing units (GPUs) 815. The processor 810and/or the GPU 815 may include integrated memory and/or be incommunication with system memory 820. The processor 810 and/or the GPU815 may be a specialized microprocessor, such as a digital signalprocessor (DSP), a very long instruction word (VLIW) processor, or othermicrocontroller, or may be a general purpose central processing unit(CPU) having one or more processing cores. The processor 810, the GPU815, the system memory 820, and/or any other components of the computingenvironment 800 may be packaged or otherwise integrated as a system on achip (SoC), application-specific integrated circuit (ASIC), or otherintegrated circuit or system.

The computing environment 800 may also include other components, suchas, for example, a communications interface 830. One or more computerinput devices 840 (e.g., pointing devices, keyboards, audio inputdevices, video input devices, haptic input devices, devices forreceiving wired or wireless data transmissions, etc.) may be provided.The input devices 840 may include one or more touch-sensitive surfaces,such as track pads. Various output devices 850, including touchscreen ortouch-sensitive display(s) 855, may also be provided. The output devices850 may include a variety of different audio output devices, videooutput devices, and/or devices for transmitting wired or wireless datatransmissions.

The computing environment 800 may also include a variety of computerreadable media for storage of information such as computer-readable orcomputer-executable instructions, data structures, program modules, orother data. Computer readable media may be any available mediaaccessible via storage devices 860 and includes both volatile andnonvolatile media, whether in removable storage 870 and/or non-removablestorage 880.

Computer readable media may include computer storage media andcommunication media. Computer storage media may include both volatileand nonvolatile, removable and non-removable media implemented in anymethod or technology for storage of information such as computerreadable instructions, data structures, program modules or other data.Computer storage media includes, but is not limited to, RAM, ROM,EEPROM, flash memory or other memory technology, CD-ROM, digitalversatile disks (DVD) or other optical disk storage, magnetic cassettes,magnetic tape, magnetic disk storage or other magnetic storage devices,or any other medium which may be used to store the desired informationand which may accessed by the processing units of the computingenvironment 800.

The localized backlighting techniques described herein may beimplemented in computer-executable instructions, such as programmodules, being executed by the computing environment 800. Programmodules include routines, programs, objects, components, datastructures, etc., that perform particular tasks or implement particularabstract data types. The techniques described herein may also bepracticed in distributed computing environments where tasks areperformed by one or more remote processing devices, or within a cloud ofone or more devices, that are linked through one or more communicationsnetworks. In a distributed computing environment, program modules may belocated in both local and remote computer storage media including mediastorage devices.

The techniques may be implemented, in part or in whole, as hardwarelogic circuits or components, which may or may not include a processor.The hardware logic components may be configured as Field-programmableGate Arrays (FPGAs), Application-specific Integrated Circuits (ASICs),Application-specific Standard Products (ASSPs), System-on-a-chip systems(SOCs), Complex Programmable Logic Devices (CPLDs), and/or otherhardware logic circuits.

The technology described herein is operational with numerous othergeneral purpose or special purpose computing system environments orconfigurations. Examples of well-known computing systems, environments,and/or configurations that may be suitable for use with the technologyherein include, but are not limited to, personal computers, hand-held orlaptop devices, mobile phones or devices, multiprocessor systems,microprocessor-based systems, set top boxes, programmable consumerelectronics, network PCs, minicomputers, mainframe computers,distributed computing environments that include any of the above systemsor devices, and the like.

The technology herein may be described in the general context ofcomputer-executable instructions, such as program modules, beingexecuted by a computer. Generally, program modules include routines,programs, objects, components, data structures, and so forth thatperform particular tasks or implement particular abstract data types.The technology herein may also be practiced in distributed computingenvironments where tasks are performed by remote processing devices thatare linked through a communications network. In a distributed computingenvironment, program modules may be located in both local and remotecomputer storage media including memory storage devices.

In one aspect, a backlight unit includes a waveguide assembly includinga first wedge and a birefringent wedge disposed adjacent to, andarranged nose-to-tail with, the first wedge, to define an interface ofthe waveguide assembly. The birefringent wedge has different indices ofrefraction for light propagating through the waveguide assembly in firstand second polarization states. The first wedge is configured topropagate the light in the second polarization state at a differentspeed than the birefringent wedge. The backlight unit further includes aliquid crystal layer extending along the waveguide assembly andconfigured to selectively switch the light between the first and secondpolarization states. The backlight unit still further includes anextraction grating extending laterally along the waveguide assembly andconfigured to extract the light from the waveguide assembly at anemission angle established in accordance with an angle of incidence ofthe light on the extraction grating. The angle of incidence isdetermined by an extent to which the light encountered the interfacewhile propagating through the waveguide assembly in the secondpolarization state.

The liquid crystal layer may include a plurality of pixels arranged in alayer having a thickness such that each pixel is configured as aquarter-wave plate when activated.

The first wedge may include an isotropic material or a birefringentmaterial. In the latter case, the first wedge and the birefringent wedgemay have first and second axes of birefringence, respectively. The firstand second axes of birefringence may be orthogonal to one another.

In some cases, the backlight unit further includes a panel waveguidedisposed adjacent to the waveguide assembly, extending over a viewabledisplay area, and including a plurality of volume holograms responsiveto the emission angle at which the light is extracted from the waveguideassembly to determine a further emission angle at which the light isemitted from the panel waveguide. The plurality of volume holograms maybe configured to amplify an extent to which the light extracted from thewaveguide assembly is converging. Alternatively or additionally, theplurality of volume holograms may include an array of volume hologramsdistributed over a viewable display area. Alternatively or additionally,the waveguide assembly may be disposed at an end region of the panelwaveguide outside of the viewable display area. Alternatively oradditionally, the panel waveguide may include a set of volume hologramsdistributed laterally across the panel waveguide in the end region, inwhich case the waveguide assembly may be disposed at the end region suchthat the light extracted at the emission angle is emitted into the panelwaveguide for interaction with the set of volume holograms. The set ofvolume holograms may redirect the light toward the array of volumeholograms at a panel propagation angle based on the emission angle atwhich the light is injected into the panel waveguide. Each volumehologram of the array may be configured to emit backlight from the panelwaveguide at a convergence angle in accordance with the panelpropagation angle and the emission angle.

In one aspect, a display includes a backlight unit, a liquid crystallayer, an extraction grating, a liquid crystal display (LCD) assembly, acamera, and a processor. The backlight unit includes a light source, awaveguide assembly disposed adjacent the light source to receive lightfrom the light source and including a first wedge and a birefringentwedge disposed adjacent to, and arranged nose-to-tail with, the firstwedge, to define an interface of the waveguide assembly. Thebirefringent wedge has different indices of refraction for lightpropagating through the waveguide assembly in first and secondpolarization states. The first wedge is configured to propagate thelight in the second polarization state at a different speed than thebirefringent wedge. The liquid crystal layer extends along the waveguideassembly and is configured to selectively switch the light between thefirst and second polarization states as the light reflectivelypropagates through the waveguide assembly. The extraction gratingextends laterally along the waveguide assembly and is configured toextract the light from the waveguide assembly at emission anglesestablished in accordance with respective angles of incidence of thelight on the extraction grating determined by an extent to which thelight encountered the interface while propagating through the waveguideassembly in the second polarization state. The LCD assembly isconfigured to form images and is disposed relative to the backlight unitfor illumination by the light extracted from the waveguide assembly. Thecamera is configured to capture camera data of a viewer of the display.The processor is coupled to the camera to determine, based on the cameradata, data indicative of pupil location for the viewer. The processor isfurther coupled to the backlight unit to, based on the data indicativeof the pupil location, control an angle at which the light sourceinjects the light into the waveguide assembly to adjust directionalityof the illumination provided by the backlight unit, and selectivelyactivate pixels of the liquid crystal layer to adjust convergence of theillumination. In some cases, the backlight unit further includes a panelwaveguide as described above.

In one aspect, a display includes a backlight unit and a processorcoupled to the backlight unit. The backlight unit includes a lightsource, an elongate waveguide assembly having an end disposed adjacentthe light source to receive light from the light source, a panelwaveguide adjacent the elongate waveguide assembly, extending across anentire viewable area of the display, having an edge along which theelongate waveguide assembly extends laterally across the panelwaveguide, and through which illumination from the backlight unit isprovided, a liquid crystal layer extending along the elongate waveguideassembly and configured to selectively switch the light between firstand second polarization states as the light reflectively propagatesthrough the waveguide assembly, and an extraction grating extendinglaterally along the elongate waveguide assembly and configured toextract the light from the elongate waveguide assembly. The elongatewaveguide assembly includes a first wedge and a birefringent wedgedisposed adjacent to, and arranged nose-to-tail with, the first wedge,to define an interface of the waveguide assembly. The birefringent wedgehas different indices of refraction for light propagating through thewaveguide assembly in first and second polarization states. The firstwedge is configured to propagate the light in the second polarizationstate at a different speed than the birefringent wedge. The extractiongrating extracts the light at emission angles established in accordancewith respective angles of incidence of the light on the extractiongrating determined by an extent to which the light encountered theinterface while propagating through the elongate waveguide assembly inthe second polarization state. The processor is configured to control anangle at which the light source injects the light into the elongatewaveguide assembly to adjust directionality of illumination provided bythe backlight unit, and selectively activate pixels of the liquidcrystal layer to adjust convergence of the illumination. The panelwaveguide includes a plurality of volume holograms responsive to theemission angles at which the light is extracted from the elongatewaveguide assembly to determine further emission angles at which theillumination is emitted from the panel waveguide. In some cases, theelongate waveguide is disposed along a front face of the panel waveguideor along a rear face of the panel waveguide.

While the present invention has been described with reference tospecific examples, which are intended to be illustrative only and not tobe limiting of the invention, it will be apparent to those of ordinaryskill in the art that changes, additions and/or deletions may be made tothe disclosed embodiments without departing from the spirit and scope ofthe invention.

The foregoing description is given for clearness of understanding only,and no unnecessary limitations should be understood therefrom, asmodifications within the scope of the invention may be apparent to thosehaving ordinary skill in the art.

What is claimed is:
 1. A backlight unit comprising: a waveguide assemblycomprising a first wedge and a birefringent wedge disposed adjacent to,and arranged nose-to-tail with, the first wedge, to define an interfaceof the waveguide assembly, wherein: the birefringent wedge has differentindices of refraction for light propagating through the waveguideassembly in first and second polarization states; and the first wedge isconfigured to propagate the light in the second polarization state at adifferent speed than the birefringent wedge; and a liquid crystal layerextending along the waveguide assembly, the liquid crystal layer beingconfigured to selectively switch the light between the first and secondpolarization states.
 2. The backlight unit of claim 1, wherein theliquid crystal layer comprises a plurality of pixels arranged in a layerhaving a thickness such that each pixel is configured as a quarter-waveplate when activated.
 3. The backlight unit of claim 1, wherein thefirst wedge comprises an isotropic material.
 4. The backlight unit ofclaim 1, wherein: the first wedge comprises a birefringent material; thefirst wedge and the birefringent wedge have first and second axes ofbirefringence, respectively; and the first and second axes ofbirefringence are orthogonal to one another.
 5. The backlight unit ofclaim 1, wherein both of the refractive indices of the birefringentwedge are offset from a refractive index of the first wedge.
 6. Thebacklight unit of claim 1, wherein one of the refractive indices of thebirefringent wedge is about equal to a refractive index of the firstwedge.
 7. The backlight unit of claim 1, further comprising a panelwaveguide disposed adjacent to the waveguide assembly, extending over aviewable display area, and comprising a plurality of volume hologramsresponsive to an emission angle at which the light is extracted from thewaveguide assembly to determine a further emission angle at which thelight is emitted from the panel waveguide.
 8. The backlight unit ofclaim 7, wherein the plurality of volume holograms are configured toamplify an extent to which the light extracted from the waveguideassembly is converging.
 9. The backlight unit of claim 7, wherein theplurality of volume holograms comprises an array of volume hologramsdistributed over a viewable display area.
 10. The backlight unit ofclaim 7, wherein the waveguide assembly is disposed at an end region ofthe panel waveguide outside of the viewable display area.
 11. Thebacklight unit of claim 10, wherein: the panel waveguide comprises a setof volume holograms distributed laterally across the panel waveguide inthe end region; and the waveguide assembly is disposed at the end regionsuch that the light extracted at an emission angle is emitted into thepanel waveguide for interaction with the set of volume holograms; theset of volume holograms redirect the light toward the array of volumeholograms at a panel propagation angle based on the emission angle atwhich the light is injected into the panel waveguide; and each volumehologram of the array is configured to emit backlight from the panelwaveguide at a convergence angle in accordance with the panelpropagation angle and the emission angle.
 12. A display comprising: abacklight unit comprising: a light source; a waveguide assembly disposedadjacent the light source to receive light from the light source andcomprising a first wedge and a birefringent wedge disposed adjacent to,and arranged nose-to-tail with, the first wedge, to define an interfaceof the waveguide assembly, wherein: the birefringent wedge has differentindices of refraction for light propagating through the waveguideassembly in first and second polarization states; and the first wedge isconfigured to propagate the light in the second polarization state at adifferent speed than the birefringent wedge; a liquid crystal layerextending along the waveguide assembly, the liquid crystal layer beingconfigured to selectively switch the light between the first and secondpolarization states as the light reflectively propagates through thewaveguide assembly; a liquid crystal display (LCD) assembly configuredto form images, the LCD assembly being disposed relative to thebacklight unit for illumination by the light extracted from thewaveguide assembly; a camera to capture camera data of a viewer of thedisplay; and a processor coupled to the camera to determine, based onthe camera data, data indicative of pupil location for the viewer;wherein the processor is further coupled to the backlight unit to, basedon the data indicative of the pupil location, control an angle at whichthe light source injects the light into the waveguide assembly to adjustdirectionality of the illumination provided by the backlight unit, andselectively activate pixels of the liquid crystal layer to adjustconvergence of the illumination.
 13. The display of claim 12, whereinthe backlight unit further comprises a panel waveguide disposed adjacentto the waveguide assembly, extending across a viewable area of thedisplay, through which the illumination is provided, and comprising aplurality of volume holograms responsive to emission angles at which thelight is extracted from the waveguide assembly to determine furtheremission angles at which the illumination is emitted from the panelwaveguide.
 14. The display of claim 13, wherein the plurality of volumeholograms are configured to amplify an extent to which the lightextracted from the waveguide assembly is converging.
 15. The display ofclaim 13, wherein the plurality of volume holograms comprises an arrayof volume holograms distributed across a viewable display area.
 16. Thedisplay of claim 13, wherein the waveguide assembly is disposed at anend region of the panel waveguide outside of the viewable display area.17. The display of claim 16, wherein: the panel waveguide comprises aset of volume holograms distributed laterally across the panel waveguidein the end region; and the waveguide assembly is disposed at the endregion such that the light extracted at an emission angle is emittedinto the panel waveguide for interaction with the set of volumeholograms; the set of volume holograms redirect the light toward thearray of volume holograms at a panel propagation angle based on theemission angle at which the light is injected into the panel waveguide;and the array of volume holograms is configured to emit backlight fromthe panel waveguide at a convergence angle in accordance with theemission angle.
 18. The display of claim 12, wherein: the first wedgecomprises a birefringent material; the first wedge and the birefringentwedge have first and second axes of birefringence, respectively; and thefirst and second axes of birefringence are orthogonal to one another.19. A display comprising: a backlight unit comprising: a light source;an elongate waveguide assembly having an end disposed adjacent the lightsource to receive light from the light source; a panel waveguideadjacent the elongate waveguide assembly, extending across an entireviewable area of the display, having an edge along which the elongatewaveguide assembly extends laterally across the panel waveguide, andthrough which illumination from the backlight unit is provided; a liquidcrystal layer extending along the elongate waveguide assembly, theliquid crystal layer being configured to selectively switch the lightbetween first and second polarization states as the light reflectivelypropagates through the waveguide assembly; and an extraction gratingextending laterally along the elongate waveguide assembly, theextraction grating being configured to extract the light from theelongate waveguide assembly; wherein: the elongate waveguide assemblycomprises a first wedge and a birefringent wedge disposed adjacent to,and arranged nose-to-tail with, the first wedge, to define an interfaceof the waveguide assembly; the birefringent wedge has different indicesof refraction for light propagating through the waveguide assembly infirst and second polarization states; and the first wedge is configuredto propagate the light in the second polarization state at a differentspeed than the birefringent wedge; the extraction grating extracts thelight at emission angles established in accordance with respectiveangles of incidence of the light on the extraction grating; a processorcoupled to the backlight unit to control an angle at which the lightsource injects the light into the elongate waveguide assembly to adjustdirectionality of illumination provided by the backlight unit, theprocessor being configured to selectively activate pixels of the liquidcrystal layer to adjust convergence of the illumination; wherein thepanel waveguide comprises a plurality of volume holograms responsive tothe emission angles at which the light is extracted from the elongatewaveguide assembly to determine further emission angles at which theillumination is emitted from the panel waveguide.
 20. The display ofclaim 19, wherein the elongate waveguide is disposed along a front faceof the panel waveguide or along a rear face of the panel waveguide.